This invention pertains to improved three-way precious metal catalysts which utilize a composite oxygen-ion conducting support material and provide treatment for gas emissions or pollutants from fuel combustion sources. It pertains particularly to such catalysts in which the oxygen-ion conducting support material is provided by a composite of an oxygen-ion conducting material and an inert support material on which at least two chemically active metals are dispersed, for simultaneous oxidation of CO, H.sub.2 and hydrocarbon compounds and reduction of nitrogen oxides present in fuel combustion gases. The oxygen-ion conducting support material can be zirconia stabilized by yttria, calcia, magnesia, or scandia and the inert material can be alumina or titania, so as to provide improved thermal stability and increased surface area and activity for the catalyst.
Supported metal catalysts are widely used for oxidation and reduction of pollutants present in exhaust gas emissions from mobile or stationary combustion power generation sources, such as automotive exhaust gases or power plant stack gas emissions. One approach to solving this gas emission pollutant problem by simultaneous oxidation of carbon monoxide and hydrocarbons and reduction of NO.sub.x compounds in exhaust gases is passing such gases through a single bed catalytic converter. Three-way catalysts used in such catalytic converters typically consist of platinum (Pt), rhodium (Rh) and sometimes palladium (Pd) deposited on a conventional .gamma.-alumina (Al.sub.2 O.sub.3) support material, which may be in either pellet or a monolithic form.
The existing precious metal catalysts used for treatment of automotive exhaust gas emissions typically use .gamma.-alumina and sometimes ceria-stabilized and/or lanthanum-oxide-stabilized .gamma.-alumina as a support material. The addition of ceria may promote the performance of the catalyst by facilitating a water-gas shift reaction, by acting as an oxygen storage unit, and by stabilizing the gamma-alumina support against loss of surface area at high temperatures. Also, known catalyst support materials used in catalysts for automotive exhaust gas emissions treatment can advantageously have an oxygen-ion conducting property, as was disclosed in our co-pending patent application Ser. No. 07/459,560, which is incorporated herein by reference. We have observed that such catalysts having yttria-stabilized-zirconia (YSZ) supports used alone provide desired thermal stability and moderate useful active life for the catalyst. However, to provide catalysts having an improved substrate or support material particularly for use in automotive catalytic converters requiring long active useful life, the support material must have high thermal stability and surface area exceeding about 20 m.sup.2 /gm after extended thermal aging or service use, because in automotive service the catalysts can be subjected to prolonged temperatures as high as 1000.degree. C.
We have now discovered that improved stable catalysts utilizing an oxygen-ion conducting catalyst support material can be advantageously made using a composite of an oxygen-ion conducting material and an inert support material, such as composites of yttria-stabilized-zirconia (YSZ) and at least 40 wt. % inert support material such as alumina or titania. Such composite support materials provide for improved precious metal catalysts having enhanced pollutant removal capability and high thermal stability, without the need for using high percentages of the oxygen-ion conducting support material or active precious metals content of the catalyst. These three-way composite catalysts utilizing e.g. Pt/Rh active metals on YSZ-Al.sub.2 O.sub.3 support material are significantly superior to catalysts using Pt/Rh active metals deposited on conventional supports such as -alumina stabilized with cerium oxide or lanthanum oxide, and provide improved thermal stability and longer catalyst life.